Sawtooth structure with reversed cutting function and its drill series

ABSTRACT

A kind of sawtooth structure with reversed cutting function and its drill series are specifically fitted for drilling carbon fiber reinforced polymers (CFRP). The sawtooth structure is constructed by sawteeth and grooves located on the cutting edges of drill. And the sawtooth structure is helically arranged with a large helix angle around the axis of the drill. It can be divided into left-hand sawtooth structure and right-hand sawtooth structure by the helix angle. The bottom edges of the left-hand sawtooth function as cutting edges, and the rake angles are positive, while the top edges of the right-hand sawtooth are ground as positive rake angles. The sawtooth structure can be applied into multiple drilling tools, which proves to be universal. And it can effectively reduce delamination at both hole entry and hole exit as well as burrs defects. The series of drilling tools are double point angle drill, twist drill and stepped drill with sawtooth structure on the cutting edges. And such forms of drilling tools can achieve hole drilling of CFRP with minimum drilling defects. The drilling performance of the tools is thus improved to a great extent, and the tool life is extended while maintaining low usage costs.

TECHNICAL FIELD

The invention belongs to the drilling tool technical field in the mechanical machining, and involves a type of sawtooth structure with reversed cutting function and its drill series. And the invention is specifically fitted for drilling carbon fiber reinforced polymers with one shot to guarantee high quality and high drilling efficiency.

TECHNICAL BACKGROUND

The fiber reinforced polymer (FRP) generally have excellent mechanical properties, and its overall property can be designable and it is easy to be manufactured to the net shape. Therefore the FRP is widely used in the manufacturing of structural parts in aeronautical field and effectively reduces the weight of structural parts and the manufacturing cycle as well as improves reliability. It is especially true for carbon fiber reinforced polymer (CFRP) due to its high strength and high rigidity, and CFRP is usually applied for manufacturing core load-bearing structures. However, machining is necessary in order to achieve the assembly among all structural parts. And hole-drilling is one of the most heavy-tasked work in the whole process. For example, there are more than three million holes on one single Boeing 747. Due to the fact that CFRP is comprised of carbon fiber reinforcement and resin matrix, the material is anisotropic and stacked, and the bonding between two adjacent laminates is weak. The drill point easily induces relatively large thrust force because of weak support at both hole entry and hole exit, and the thrust force can break the outermost laminates and induces delaminations as well as fiber pullouts and burrs. At the same time, the extremely abrasive carbon fiber can cause rapid tool wear and thus dull the cutting edges, which leads to difficulty in cutting off carbon fibers and subsequent burrs at hole entry and hole exit. In order to overcome above issues and achieve high quality drilling of CFRP, companies are applying diamond coating to the drill bits and utilizing multiple process including ‘drilling-boring-reaming’ to ensure the final quality.

The geometrical structures of drill are the major influencing factor of the CFRP machining quality in the scientific researches. Therefore, there are many drills with different structures develop both domestically and overseas for drilling CFRP. The geometrical structures have their own benefits and drawbacks. For example, in patent 201310104178.6 issued by Suzhou Anuo precision machining company, the drill bit has two flutes, and the structure forms two primary cutting edges with intersecting the drill axis. The characteristics of the structures are stated as three points, and one point is lower than the other two, thus the structure can guarantee the strength of the drill bit and centering capability. And the two outer points can effectively cutting off carbon fibers with less burrs. However, fiber pullouts are easily generated and tool wear occurs rapidly in drilling laminated materials with low bonding. And severe tool wear can lead to significant quality drop of drilled holes. The Sandvik Company reveals a patent (201180020931.2) that depicts a twist drill for drilling advanced materials. It can be applied for drilling such as carbon fiber reinforced polymers and glass fiber reinforced polymers. The helix angle are variable within the drill bit, and the initial and final helix angles are fixed. And by combining primary and minor relief angles, the thrust force can be minimized. The specially designed twist drill can reduce delaminations to a certain extent, however, the tool life is relatively short thus leading to burrs rapidly.

Invention Disclosure

This invention relates to a method of solving the technical problem that defects, such as delamination, burrs and cracks, easily generate at the hole exit and entry in drilling fiber reinforced composite.

The technical scheme adopted by the invention is:

The sawtooth structure is constructed by sawteeth 1-1 and grooves 1-2 located on the cutting edges of drill; The sawteeth 1-1 and grooves 1-2 are helically arranged around the drill axis; The sawtooth structure can be divided into left-hand sawtooth structure and right-hand sawtooth structure by the sawtooth helix angle 1-β; The sawteeth 1-1 with acute helix angle 1-β larger than 60° are categorized as right-hand sawteeth, and their top edges 1-3 perform as cutting edges that are ground into positive rake angles 1-γ_(0u); The sawteeth 1-1 with right angle or obtuse helix angle 1-β less than 150° are categorized as left-hand sawteeth, and their bottom edges 1-4 perform as cutting edges that are ground into positive rake angles 1-γ_(0d); The sawtooth structure is located on the cutting edges whose diameters are equal to 0.6 to 1 times of drill diameter, and for stepped drilling structures the sawtooth structure is located at each stepped minor cutting edges; The length of sawteeth 1-L_(t) is less than the length of drilling margin while maintaining at least 0.8 mm; The width of grooves 1-W_(g) is larger than the thickness of two CFRP laminates and at the same time maintains at least 0.3 mm; The width of sawteeth 1-W_(t) is at least 0.5 mm; The depth of grooves 1-D_(t) is at least 0.8 mm, and the sawteeth at minimum diameter cannot interfere with the drilling core.

The double point angle drill with the above intersecting sawtooth structure has characteristics as below: The right-hand and left-hand sawteeth are located at the cutting edges of the double point angle drill with four flutes; The sawteeth cutting zone 2-E is arranged at the end of four secondary cutting edges 2-2 within the secondary cutting zone 2-B as well as at the front of four minor cutting edges 2-3 within the minor cutting zone 2-C; The micro teeth cutting zone 2-E is constructed by sawtooth structure arranged at separate cutting edges; The profiles of sawteeth 1-1 and grooves 1-2 remain the same within one cutting edge.; The number of sawteeth along one cutting edge is 3-7, and the number is identical within all cutting edges; The helix angles of sawteeth 1-1 and grooves 1-2 are different within two adjacent cutting edges while the helix angles of sawteeth 1-1 and grooves 1-2 are identical within two opposite cutting edges; The helix angle 2-β_(R) of sawteeth 1-1 on one set of cutting edges are right-hand acute angles larger than 60°, and the top edges 1-3 perform as cutting edges which are ground into positive rake angles 1-γ_(0u); The helix angle 2-β_(L) of sawteeth 1-1 on the other set of cutting edges are left-hand right angle or obtuse angles less than 150°, and the bottom edges 1-4 perform as cutting edges which are ground into positive rake angles 1-γ_(0d); The above stated right-hand and left-hand sawteeth form the intersecting sawteeth cutting zone 2-E on two adjacent cutting edges.

The right-hand double point angle drill with the above sawtooth structure has characteristics as below: The right-hand sawteeth are arranged helically on the cutting edges of the drill with four flutes, and the sawteeth cutting zone 3-E is arranged at the front of the four secondary cutting edges at the secondary cutting zone 3-B and the end of the four minor cutting edges at the minor cutting zone 3-C. The sawteeth cutting zone 3-E is constructed by sawtooth structure located at separated cutting edges; The number of sawteeth 1-1 on the secondary cutting edge is 1 to 3 and the number of sawteeth 1-1 on the minor cutting edge is 2 to 4; The sawteeth helix angle 1-β of all the sawteeth 1-1 is acute angle larger than 60° as well as right-handed, and top edges 1-3 on the sawteeth perform as cutting edges that are ground into positive rake angles 1-γ_(0u).

The right-hand twist drill with the above sawtooth structure has characteristics as below: The right-hand sawteeth are arranged on the minor cutting edges; The point angle 4-α of the drill is 85°-90°, and the length of the chisel edge is 0.1-0.6 mm. The rake angle 4-γ₀ of the primary cutting edges is ground into fixed value and the rake angle 4-γ₀ is 15°-20°. The first flank angle 4-α₁ on the primary cutting edges 4-1 is 10°-12° and the second flank angle 4-α₂ on the primary cutting edges 4-1 is 15°-25°. The area of the second flank face 4-8 is 3 to 5 times larger than the area of the first flank face 4-7; The helix angle of the flutes is 30°-34°; The sawteeth cutting zone 4-D is located at the minor cutting edges 4-2 within the minor cutting zone 4-B; The number of sawteeth 1-1 along every minor cutting edge 4-2 is 3-5 and the number is identical among all the minor cutting edges 4-2 as well; The distance 4-d between the sawteeth closest to the drill bit and the primary cutting zone 4-A is 1-1.5 mm; The sawteeth 1-1 and the grooves 1-2 are helically arranged around the drill axis 4-4 and the sawteeth helix angle 1-β is acute angle larger than 60°; The top edges 1-3 of the sawteeth perform as cutting edges that are ground into positive rake angles 1-γ_(0u).

The right-hand stepped drill with the above sawtooth structure has characteristics as below: The right-hand sawteeth are arranged on each stepped minor cutting edges of the stepped drill; The stated stepped cutting zone 5-A is constructed by stepped structures of n steps and n is equal to or larger than 2; Each stepped structure is comprised of primary stepped cutting zone and minor stepped cutting zone; The stated sawteeth cutting zone 5-D is located on the stepped cutting zone 5-A and the sawteeth cutting zone 5-D is constructed by sawtooth structure with n steps; The n steps are arranged in the direction from drill point to clamping zone; There are sawteeth structure of Step j on the stepped minor cutting zone of Step j, and j is equal to or larger than 1 while less or equal to than n.

The tolerance of the above right-hand stepped drill with sawtooth structure between the nominal diameter of the Step j of n steps and the nominal diameter of the Step j-1 is 20% to 40% of the nominal diameter of the Step j, and the j is equal to or larger 2 while equal to or less than n; When j is larger than 2, the tolerance is set at 20% of the nominal diameter of the Step j; The axial length of the stepped minor cutting edges of each stepped structure should be guaranteed to accommodate at least 2 sawteeth; When j equals 1, the point angle of Step j is set at 80°-100°; When j is equal to or larger than 2, the point angle of Step j is set at 60°-80°, and the point angle of Step j is smaller than that of Step j-1; The rake angle of primary cutting edges of each stepped structure is ground into fixed value; The margin length of each minor cutting edge is 0.5-1.5 mm; The tool margins on the minor cutting edges of stepped structures are ground into L shapes.

There are identical sawteeth within sawteeth cutting zone 5-D of each stepped structure, and the number of micro teeth stays at 2-4; The axial distance between the sawteeth closest to the drill point on each stepped structure and the end of corresponding primary cutting edge is set at 1-1.5 mm; When j is larger than 1 and less than or equal to n, the ratio between width of the grooves of Step j-1 and that of Step j is 0.6-0.8. Additionally, the ratio between the width of sawteeth of Step j-1 and that of Step j is 0.5-0.7. When j equals to 2, the width of micro teeth of Step 1 is at least 0.5 mm, and the width of grooves is at least 0.3 mm; The sawteeth 1-1 and the grooves 1-2 are helically arranged around the drill axis, and the helix angle of sawteeth is acute angle larger than 60°. The bottom edges 1-3 of sawteeth perform as cutting edges, and the rake angle of the edges is ground into positive angle 1-γ_(0u).

The effects and benefits of the invention are as follows: The sawtooth structure with reverse cutting function and its drill series, the sawtooth structure is helically arranged with a large helix angle around the axis of the drill. In drilling fiber reinforced composites, the sawtooth structure has the recutting function at the hole entry and the reversed cutting function in the direction opposite to the main cutting movement at the hole exit. The sawtooth structure can be applied into multiple drilling tools, which proves to be universal. And it can effectively reduce delamination at both hole entry and hole exit as well as burrs defects; The drill series with the sawtooth structure can make the burr rebound into the sawtooth structure at the exit and entry in different working conditions. It also has the advantages of cutting burrs, which rebound into the sawtooth structure, cutting off them in the downward drilling process, thereby effectively removing burrs at the exit and entry. The invention reduces the effect of the cracks in the final hole, and greatly improves the cutting ability of the drill.

FIGURE LEGENDS

FIG. 1 (a): the front view of the right-hand double point angle drill with sawtooth structure.

FIG. 1 (b): the side view of the right-hand double point angle drill with sawtooth structure.

FIG. 2: the front view of the double point angle drill with intersecting sawtooth structure.

FIG. 3 (a): the local enlarged figure of the right-hand sawtooth structure of the sawteeth cutting zone 2-E in FIG. 2

FIG. 3 (b): the local enlarged figure of the left-hand sawtooth structure of the sawteeth cutting zone 2-E in FIG. 2

FIG. 4: the front view of the right-hand twist drill with the sawtooth structure.

FIG. 5: the local enlarged figure of the primary cutting zone 4-A and the sawteeth cutting zone 4-D in FIG. 4.

FIG. 6: the upward view of the right-hand twist drill with the sawtooth structure.

FIG. 7: the front view of right-hand stepped drill with sawtooth structure.

FIG. 8: the local enlarged figure of the stepped cutting zone 5-A in FIG. 7.

FIG. 9 (a): cutting diagrammatic sketch of the right-hand sawtooth structure.

FIG. 9 (b): cutting diagrammatic sketch of the left-hand sawtooth structure.

FIG. 10 (a): the hole entry machined by drill #1.

FIG. 10 (b): the hole exit machined by drill #1.

In Figure:

1-1 sawteeth, 1-2 grooves, 1-3 the top edges of the sawteeth, 1-4 the bottom edges of the sawteeth, 1-β the helix angle of the sawtooth structure, 1-W_(t) the width of sawteeth, 1-W_(g) the width of grooves, 1-D_(t) the depth of grooves, 1-L_(t) the length of sawteeth cutting edges, 1-γ_(0u) the top rake angle ; 1-γ_(0d) the bottom rake angle; 2-A the primary cutting zone, 2-B the secondary cutting zone, 2-C the minor cutting zone, 2-D the clamping zone, 2-E the sawteeth cutting zone, 2-1 the primary cutting edge, 2-2 the secondary cutting edge, 2-3 the minor cutting edge, 2-4 the helical line of the sawteeth, 2-α the first point angle, 2-β the second point angle, 2-D₁ the nominal diameter of the drill, 2-β_(L) the helix angle of the left-hand sawtooth structure, 2-β_(R) the helix angle of the right-hand sawtooth structure; 3-A the primary cutting zone, 3-B the secondary cutting zone, 3-C the minor cutting zone, 3-D the clamping zone, 3-E the sawteeth cutting zone, 3-1 the primary cutting edge, 3-2 the secondary cutting edge, 3-3 the minor cutting edge, 3-α the first point angle, 3-β the second point angle, 3-D₁ the nominal diameter of the drill, 3-e the maximum width of the two primary cutting edges; 4-A the primary cutting zone, 4-B the minor cutting zone, 4-C the clamping zone, 4-D the sawteeth cutting zone, 4-1 the main cutting edge, 4-2 the minor cutting edge, 4-3 the end of the minor cutting edge, 4-4 the drill axis, 4-5 the axis of the sawtooth, 4-6 the chisel edge, 4-7 the first flank face, 4-8 the second flank face, 4-α the point angle, 4-γ₀ the rake angle of the main cutting edge, 4-α₁ the first flank angle, 4-α₂ the second flank angle, 4-d the distance between the sawteeth closest to the drill bit and the end of primary cutting zone; 5-A the stepped cutting zone, 5-B the minor cutting zone, 5-C the clamping zone, 5-D the sawteeth cutting zone, 5-I the first stepped structure, 5-II the second stepped structure, 5-i the first sawtooth structure, 5-ii the second sawtooth structure, 5-1 the primary cutting edge, 5-2 the secondary cutting edge, 5-3 the minor cutting edge, 5-4 inverted L shape structure, 5-5 the drill axis, 5-6 the axis of the sawtooth, 5-α₁ the point angle of the first stepped structure, 5-α₂ the point angle of the second stepped structure, 5-a the axial length of the first stepped structure, 5-b the axial length of the second stepped structure, 5-c the margin width of the minor cutting edge, 5-D₁ the nominal diameter of the first stepped structure, 5-D₂ the nominal diameter of the second stepped structure.

Specific Implementation Way Optimum Implementation

FIG. 9 is the schematic diagram of the sawtooth structure protected by claim 1. It can be seen that the burr F at the exit and entry will rebound into grooves 1-2 in drilling, as shown in FIG. 9(a) and FIG. 9(b). Under the condition that the rotating speed is much higher than the feed rate , the helix angle 1-β of right-hand sawteeth is an acute angle that is bigger than 60° at the exit. The top edge 1-3 has a movement relative to the exit Ex, which forms the scissor-like structure and reversely shears burr F. At the entry, the helix angle 1-β of left-hand sawteeth is an obtuse angle that is smaller than 150°. The bottom edge 1-4 has a movement relative to the entrance En, which also forms the scissor-like structure and reversely shears burr F.

Implementation 1

FIG. 2 shows the double point angle drill with intersecting sawtooth structure which is protected by claim 2. The drill can be divided into five zones: the primary cutting zone 2-A formed by the envelope curves of primary cutting edges 2-1, the secondary cutting zone 2-B formed by the envelope curves of secondary cutting edges 2-2, the left-hand minor cutting zone 2-C whose envelope curves are final hole, the clamping zone 2-D and the sawteeth cutting zone 2-E. Among them, the primary cutting zone 2-A has two primary cutting edges 2-1, and the first point angle 2-α is obtuse, which leads to high rigidity of the primary cutting edge and preferable centering effect to ensure the drilling smoothly. The implementation adopts 2-α=118°; The secondary cutting zone 2-B has four secondary cutting edges 2-2, and the second point angle 2-β is acute, which results in the sharp secondary cutting edge 2-2, low thrust force and delamination. The minor cutting edge 2-3 adopts low left-hand helix angle to decrease thrust force and realize reaming with high rake angle. In FIG. 2, the sawteeth cutting zone 2-E distributes in four secondary cutting edges 2-2 of the secondary cutting zone 2-B; The sawteeth cutting zone 2-E consists of the sawtooth structure that distributes in each cutting edges; The sawteeth 1-1 and grooves 1-2 of the sawtooth structure have identical profile structure in the same cutting edge; There are three sawteeth 1-1 in cutting edge, and the number of them are uniform in each cutting edges. As shown in FIG. 2, FIG. 3(a) and FIG. 3(b), the helix angle of sawteeth 1-1 and grooves 1-2 are different in adjacent cutting edges, while the helix angle of sawteeth 1-1 is indentical in opposite cutting edges; Among them, The sawteeth 1-1 has the helix angle 2-β_(R) of 75° in a pair of cutting edges that are right-hand sawteeth. The top edges 1-3 of them act as cutting edge and are ground into positive rake angle 1-γ_(0u), which equals to 20°; In addition, The sawteeth 1-1 have the helix angle 2-β_(L) of 110° in another pair of cutting edges that are left-hand sawteeth. The bottom edge 1-4 of them act as cutting edges and are ground into positive rake angle 1-γ_(0d), which equals to 20°; Above right-hand sawteeth and left-hand sawteeth form the sawteeth cutting zone 2-E with crossed helix direction in adjacent cutting edges. The width of grooves 1-2 is 1.5 mm and the width of sawteeth is 1.5 times higher than that of grooves in the same secondary cutting edge. It can guarantee preferably the rigidity and avoid destruction for sawteeth in drilling.

The double point angle drill with intersecting sawtooth structure is a special structure designed for drilling of fibre reinforced composite material. It can decrease the delamination, remove burrs gradually, avoid cracks effectively at the exit and entry, and form high quality final hole in one time eventually.

Implementation 2

FIG. 1(a) and (b) shows the right-hand double point angle drill with sawtooth structure in claim 3. The drill can be divided into four zones: the primary cutting zone 3-A, the secondary cutting zone 3-B, the minor cutting zone 3-C, and clamping zone 3-D. In addition, the sawteeth cutting zone 3-E locates in the joint of the secondary cutting zone 3-B and the minor cutting zone 3-C which consists of several sawteeth 1-1 and grooves 1-2. The sawteeth cutting zone 3-E is formed by the sawtooth structure distributed in each cutting edges. There are one sawtooth 1-1 in the secondary cutting edge and four sawteeth 1-1 in the minor cutting edge. The sawteeth 1-1 in all cutting edges is right-hand. The top edges 1-3 act as cutting edges, and are ground into positive rake angle 1-γ_(0u) that equals to 20°.

Drilling can be divided into five stages: The first stage is extruding. The chisel edge contacts and extrudes the workpiece. Because of the extruding effect of it, thrust force increases rapidly; The second stage is extruding and drilling. The primary cutting edge 3-1 drills into workpiece and begins to remove material. So thrust force increase in a small extent; The third stage is drilling. The chisel edge arrives at the bottom of workpiece, and thrust force decreases suddenly after chisel edge leaving workpiece; The fourth stage is drilling and reaming. The material is drilled by the secondary cutting edge 3-2 completely and thrust force fluctuate slowly; The fifth stage is removing burrs and reaming The secondary cutting edge 3-2 starts to drill out of the material, the sawtooth structure begins to remove burrs, minor cutting edge 3-3 reams the hole, so thrust force decrease further.

The minor cutting edge 3-3 adopts left-hand thread and low helix angle. The low helix angle benefits to eliminate chip, realize reaming and decrease hole wall roughness. The drill has a clockwise rotation from the top view in drilling, and realize cutting rightward. The design of left-hand cutting edge increase the actual working rake angle of rightward cutting edge, and high rake angle is benefical to drill CFRP.

In FIG. 1(a) and (b), the nominal diameter of the drill 3-D₁ is 8 mm, and the maximum width of the two primary cutting edges 3-e is one-third of it. The main cutting zone consists of two-stage cutting edges, the primary cutting edge 3-1 and the secondary cutting edge 3-2. The first point angle 3-α formed by two primary cutting edges 3-1 and the second point angle 3-P formed by two secondary cutting edges 3-2 are different, and they develop the two-point angle structure. The first point angle 3-α adopts 90°, while the second point angle 3-P adopts 10°. The length of chisel edge need control and is shorter than drilling metal. It adopts 0.22 mm in this implementation. The number of sawteeth 1-1 is one in the secondary cutting edge 3-2, while it is four in the minor cutting edge 3-3. The width of grooves 1-W_(g) is 0.8 mm, the depth of grooves 1-D_(t) is 1.2 mm, and the width of sawteeth 1-W_(t) is 0.8 mm.

Implementation 3

FIGS. 4, 5 and 6 shows the right-hand twist drill with the sawtooth structure. It can be divided into four zones: the primary cutting zone 4-A formed by the outline of the main cutting edge 4-1, the minor cutting zone 4-B formed by the outline of the main cutting edge 4-1 terminal and minor cutting edge 4-2, the clamping zone 4-C and the sawtooth cutting zone 4-D. Two main cutting edges 4-1 develop low point angle 4-α, which is beneficial to decrease thrust force and delamination; The minor cutting edge 4-2 is designed with sawtooth structure, which realize the rebounding into sawtooth structure and cutting off for generating burrs in drilling. It can eliminate the burrs effectively at the exit and entrance, and decrease the cracks in final hole; The minor cutting edge 4-2 adopts right-hand structure, and it contributes to removing burrs; Every main cutting edges 4-1 have two flank surfaces. It will decrease the friction between drill and workpiece, cutting heat and drill wear. Eventually, the workpiece quality is improved; The changing diameter structure is used to reduce the vibration of entrance in final molding stage, and enhance the hole diameter precision and entrance quality.

The nominal diameter of the drill 4-D₁ is 8 mm, and the drill has two main cutting edges with two flank faces respectively. The rake angle of main cutting edge is 20°, the first flank angle 4-α₁ is 12°, and the second flank angle 4-α₂ is 25°. The area of second flank face 4-8 is six times bigger than that of the first flank face 4-7. The point angle 4-α is 90°, and the helix angle of minor cutting edge 4-2 is 34°.

As shown in FIG. 6, chisel edge 4-6 is formed by grinding the main cutting edges, and the chisel edge length need control. It should be smaller than the general metal drilling because smaller chisel edge contributes to reducing delamination. In addition, too small chisel edge easily leads to dropping tool life. In this implementation, the length of chisel edge 4-6 is 0.6 mm. The sawteeth cutting zone 4-D is located in the minor cutting edge near drill bit of the minor cutting zone 4-B, and consists of several sawteeth 1-1. The outline structure of sawteeth 1-1 is identical in the same minor cutting edge 4-2, which is oval; The number of sawtooth 1-1 is same in each minor cutting edges 4-2, which is three, and the axial location is also same. The distance between the sawteeth closest to the drill bit and the end of primary cutting zone 4-d is 1.5 mm. The sawteeth width is 1 mm for sawteeth 1-1 located in the same minor cutting edge 4-2, and the grooves width is 1 mm The helix angle of the sawtooth structure 1-β between sawtooth axis 4-5 and drill axis 4-4 is 85°, and the sawtooth has a spiral distribution along drill axis. The top edges of sawteeth are ground into positive rake angle 1-γ_(0u), which equal to 20°, as shown in FIG. 5.

Drilling parameters of 3000 rpm in spindle speed and 150 mm/min in feed rate are adopted in the drilling experiments for multi-directional CFRP composite with thickness of 9 mm, and all drilling operations are performed under dry condition. The inventive and purchased drill are named as Drill #1 and Drill #2 respectively. After drilling process, the quality of entrance surface and exit surface is observed. The hole quality of entrance surface and exit surface are shown in FIG. 10(a) and (b) respectively. The detailed description of experimental results is as follows:

(1) Push-Out Quality

Drill #2 has tiny delamination at the exit during drilling the first hole, and the delamination starts to aggravate with the increasing of hole number. However, Drill #1 doesn't have delamination at the exit during drilling the first hole, and its quality is still excellent until the fifth hole.

(2) Peel-Up Quality

Drill #2 has tiny delamination during drilling the first hole, and the delamination starts to aggravate with the increasing of hole number. However, Drill #1 doesn't have delamination at the entrance during drilling the first hole, and its quality is still excellent until the fifth hole.

(3) Tool Life

The delamination begins to extend in the second hole, and becomes more severe in the fourth hole for drill #2. Its tool life is very short. In addition, the fifth hole's quality is still excellent for drill #1, so drill #1 has better tool life. Implementation 4

The right-hand stepped drill with sawtooth structure in claim 1 is described in claims 5, 6 and 7, as shown in FIGS. 7 and 8. It can be divided into the stepped cutting zone 5-A, the minor cutting zone 5-B and the clamping 5-C. Among them, the stepped cutting zone 5-A consists of two stepped structure; There are two stepped structure from drill bit to the clamping zone; Each stepped structure is made up of the main stepped structure 5-I and the minor stepped structure 5-II.

The drill has the sawteeth cutting zone 5-D, which is located in the stepped cutting zone 5-A and consists of two sawteeth structure; There are 2 sawteeth structure from drill bit to the clamping zone; The j sawtooth structure is located in the j stepped structure, 1≤j≤2.

The drilling process can be divided into 10 stages: The first stage is extruding. The chisel edge contacts and extrudes workpiece, and thrust force increase rapidly because of the extruding effect for chisel edge; The second stage is extruding and drilling. The primary cutting edge 5-1 of the first stepped structure 5-I drill into the material, remove material and the thrust force increase in a low degree; The third stage is extruding, drilling and eliminating burrs for the first sawtooth structure 5-i. Meanwhile sawtooth starts to eliminate burrs; The fourth stage is extruding, drilling, eliminating burrs for the first sawteeth structure 5-i and chambering. The secondary cutting edge 5-2 of the second stepped structure 5-II drills into the material and begins to remove the material. The thrust force increase in a low extent; The fifth stage is drilling, eliminating burrs for the first sawteeth structure 5-i and chambering. At this time, the chisel edge arrives at the bottom of workpiece, and thrust force drops suddently after chisel edge drilling out of the material; The sixth stage is drilling, eliminating burrs for the first sawteeth structure 5-i, chambering and eliminating burrs for the second sawteeth structure 5-ii. The second sawteeth structure starts to eliminate burrs; The seventh stage is eliminating burrs for the first sawteeth structure 5-i, chambering, eliminating burrs for the second sawteeth structure 5-ii and reaming. At the same time, the primary cutting edge 5-1 of the first stepped structure 5-I drills out of workpiece, and the minor cutting edge 5-3 reams the hole, which leads to the further reducing of thrust force; The eighth stage is chambering, eliminating burrs for the second sawteeth structure 5-ii and reaming The first sawteeth structure drills out of the material, and the minor cutting edge 5-3 continues to ream hole; The ninth stage is eliminating burrs for the second sawteeth structure 5-ii and reaming The main cutting edge 5-2 of the second stepped structure 5-II drills out of the material, and thrust force decreases further; The tenth stage is reaming The second sawteeth structure 5-ii leaves the workpiece, and the minor cutting edge 5-3 reams the hole.

The drill adopts double stepped structure for CFRP, which contributes to reducing thrust force and delamination; The minor cutting edge 5-3 in the first stepped structure 5-I and the second stepped structure 5-II is designed with the sawtooth structure, which realize the rebounding into sawtooth structure and cutting off for generating burrs in drilling. It can eliminate the burrs effectively at the exit and entrance, and decrease the cracks in final hole; The minor cutting edge 5-3 adopts right-hand structure, and it contributes to removing burrs; The secondary cutting edges 5-2 in the second stepped structure 5-II have five flank faces. It will decrease the friction between drill and workpiece, cutting heat and drill wear. Eventually, the workpiece quality is improved; The back taper structure is used to reduce the vibration of entrance in final molding stage, and enhance the hole diameter precision and entry quality.

In this implementation, as shown in FIGS. 7 and 8, the norminal diameter 5-D₁ of the first stepped structure 5-I is 4.94 mm, while the norminal diameter 5-D₂ of the second stepped structure 5-II is 7.94 mm. So the difference of the norminal diameter between the second stepped structure 5-II and the first stepped structure 5-I is 0.38 times bigger than that of the second stepped structure 5-II; The axial length of the first stepped structure 5-a and the axial length of the second stepped structure 5-b are both 5.5 mm; the point angle of the first stepped structure 5-α₁ is 90°, while the point angle of the second stepped structure 5-α₂ is 70°; The rake angle of primary cutting edge in each stepped structures is ground into constant; The width of minor cutting edge 5-3 is 0.8 mm; The margin in the minor cutting edge of the second stepped structure 5-II near the drill bit is ground into inverted L shape structure 5-4.

The number of sawteeth in each stepped structure for the sawteeth cutting zone 5-D is identical, which is 2; The axial distance between the nearest sawtooth of each grade step structure to the drill bit and the end of main cutting edge of the stepped structure is 1 mm; For the 5-ii sawteeth structure, the sawteeth width is 0.83 mm, and the grooves width is 0.43 mm. Meanwhile, for the i sawteeth structure, the sawteeth width is 0.5 mm, and the grooves width is 0.3 mm. So the sawteeth width ratio of the i sawteeth structure to ii sawteeth structure is 0.6, and the grooves width ratio of the i sawteeth structure to ii sawteeth structure is 0.7; Sawteeth (1-1) and grooves (1-2) have a spiral design along drill axis (5-5). The helix angle are both 81° for the i sawteeth structure and ii sawteeth structure. The bottom edges (1-3) act as tooth cutting edges which are ground into positive rake angle (1-γ_(0u)) and equal to 20°.

INDUSTRIAL APPLICABILITY

The sawtooth structure with reverse shearing functions and its drill series is particularly suited to high-quality drilling for CFRP. The sawtooth structure consists of several sawtooth and groove distributed in cutting edge; The sawtooth structure has a high helix angle design along drill axis. It can be divided by the helix angle value into left-hand sawtooth and right-hand sawtooth. The bottom edges for left-hand sawtooth act as cutting edge, while the top edges for right-hand sawtooth act as cutting edge. Both of them are grinded into positive rake angle; The sawtooth structure has advantages of wide universality which can be used in several kinds of drills. It has a significant inhibition effect on exit damage and entrance exit. The sawtooth structure is applied to the cutting edge for double point angle, twist drill and step drill, which forms the new drill. The series drill can realize low-damage drilling for fiber reinforced composites, which significantly improve the drill cutting ability and service life, and reduce cost. 

1. A sawtooth structure with reversed cutting function has characteristics as below: The sawtooth structure is constructed by sawteeth (1-1) and grooves (1-2) located on the cutting edges of drill; The sawteeth (1-1) and grooves (1-2) are helically arranged around the drill axis; The sawtooth structure can be divided into left-hand sawtooth structure and right-hand sawtooth structure by the sawtooth helix angle (1-β); The sawteeth (1-1) with acute helix angle (1-β) larger than 60° are categorized as right-hand sawteeth, and their top edges (1-3) perform as cutting edges that are ground into positive rake angles (1-γ0u); The sawteeth (1-1) with right angle or obtuse helix angle (1-β) less than 150° are categorized as left-hand sawteeth, and their bottom edges (1-4) perform as cutting edges that are ground into positive rake angles (1-γ0d); The sawtooth structure is located on the cutting edges whose diameters are equal to 0.6 to 1 times of drill diameter, and for stepped drilling structures the sawtooth structure is located at each stepped minor cutting edges; The length of sawteeth (1-Lt) is less than the length of drilling margin while maintaining at least 0.8 mm; The width of grooves (1-Wg) is larger than the thickness of two CFRP laminates and at the same time maintains at least 0.3 mm; The width of sawteeth (1-Wt) is at least 0.5 mm; The depth of grooves (1-Dt) is at least 0.8 mm, and the sawteeth at minimum diameter cannot interfere with the drilling core.
 2. The double point angle drill with intersecting sawtooth structure stated in claim 1 has characteristics as below: The right-hand and left-hand sawteeth are located at the cutting edges of the double point angle drill with four flutes; The sawteeth cutting zone (2-E) is arranged at the end of four secondary cutting edges (2-2) within the secondary cutting zone (2-B) as well as at the front of four minor cutting edges (2-3) within the minor cutting zone (2-C); The sawteeth cutting zone (2-E) is constructed by sawtooth structure arranged at separate cutting edges; The profiles of sawteeth (1-1) and grooves (1-2) remain the same within one cutting edge.; The number of sawteeth along one cutting edge is 3-7, and the number is identical within all cutting edges; The helix angles of sawteeth (1-1) and grooves (1-2) are different within two adjacent cutting edges while the helix angles of sawteeth (1-1) and grooves (1-2) are identical within two opposite cutting edges; The helix angle (2-βR) of sawteeth (1-1) on one set of cutting edges are right-hand acute angles larger than 60°, and the top edges (1-3) perform as cutting edges which are ground into positive rake angles (1-γ0u); The helix angle (2-βL) of sawteeth (1-1) on the other set of cutting edges are left-hand right angle or obtuse angles less than 150°, and the bottom edges (1-4) perform as cutting edges which are ground into positive rake angles (1-γ0d); The above stated right-hand and left-hand sawteeth form the intersecting sawteeth cutting zone (2-E) on two adjacent cutting edges.
 3. The right-hand double point angle drill with sawtooth structure stated in claim 1 has characteristics as below: The right-hand sawteeth are arranged helically on the cutting edges of the drill with four flutes, and the sawteeth cutting zone (3-E) is arranged at the front of the four secondary cutting edges at the secondary cutting zone (3-B) and the end of the four minor cutting edges at the minor cutting zone (3-C). The sawteeth cutting zone (3-E) is constructed by sawtooth structure located at separated cutting edges; The number of sawteeth (1-1) on the secondary cutting edge is 1 to 3 and the number of sawteeth (1-1) on the minor cutting edge is 2 to 4; The sawteeth helix angle (1-β) of all the sawteeth (1-1) is acute angle larger than 60° as well as right-handed, and top edges (1-3) on the sawteeth perform as cutting edges that are ground into positive rake angles (1-γ0u).
 4. The right-hand twist drill with sawtooth structure stated in claim 1 has characteristics as below: The right-hand sawteeth are arranged on the minor cutting edges; The point angle (4-α) of the drill is 85°-90°, and the length of the chisel edge is 0.1-0.6 mm. The rake angle (4-γ0) of the primary cutting edges is ground into fixed value and the rake angle (4-γ0) is 15°-20°. The first flank angle (4-α1) on the primary cutting edges (4-1) is 10° -12° and the second flank angle (4-α2) on the primary cutting edges (4-1) is 15°-25°. The area of the second flank face (4-8) is 3 to 5 times larger than the area of the first flank face (4-7); The helix angle of the flutes is 30° -34° ; The sawteeth cutting zone (4-D) is located at the minor cutting edges (4-2) within the minor cutting zone (4-B); The number of sawteeth (1-1) along every minor cutting edge (4-2) is 3-5 and the number is identical among all the minor cutting edges (4-2) as well; The distance (4-d) between the sawteeth closest to the drill bit and the primary cutting zone (4-A) is 1-1.5 mm; The sawteeth (1-1) and the grooves (1-2) are helically arranged around the drill axis (4-4) and the sawteeth helix angle (1-β) is acute angle larger than 60°; The top edges (1-β) of the sawteeth perform as cutting edges that are ground into positive rake angles (1-γ0u).
 5. The right-hand stepped drill with sawtooth structure stated in claim 1 has characteristics as below: The right-hand sawteeth are arranged on each stepped minor cutting edges of the stepped drill; The stated stepped cutting zone (5-A) is constructed by stepped structures of n steps and n is equal to or larger than 2; Each stepped structure is comprised of primary stepped cutting zone and minor stepped cutting zone; The stated sawteeth cutting zone (5-D) is located on the stepped cutting zone (5-A) and the sawteeth cutting zone (5-D) is constructed by sawtooth structure with n steps; The n steps are arranged in the direction from drill point to clamping zone; There are sawteeth structure of Step j on the stepped minor cutting zone of Step j, and j is equal to or larger than 1 while less or equal to than n.
 6. The right-hand stepped drill with sawtooth structure stated in claim 5 has characteristics as below: The tolerance between the nominal diameter of the Step j of n steps and the nominal diameter of the Step j-1 is 20% to 40% of the nominal diameter of the Step j, and the j is equal to or larger 2 while equal to or less than n; When j is larger than 2, the tolerance is set at 20% of the nominal diameter of the Step j; The axial length of the stepped minor cutting edges of each stepped structure should be guaranteed to accommodate at least 2 sawteeth; When j equals 1, the point angle of Step j is set at 80°-100°; When j is equal to or larger than 2, the point angle of Step j is set at 60° -80°, and the point angle of Step j is smaller than that of Step j-1; The rake angle of primary cutting edges of each stepped structure is ground into fixed value; The margin length of each minor cutting edge is 0.5-1.5 mm; The tool margins on the minor cutting edges of stepped structures are ground into L shapes.
 7. The right-hand stepped drill with sawteeth structure stated in claim 5 has characteristics as below: There are identical sawteeth within sawteeth cutting zone (5-D) of each stepped structure, and the number of micro teeth stays at 2-4; The axial distance between the sawteeth closest to the drill point on each stepped structure and the end of corresponding primary cutting edge is set at 1-1.5 mm; When j is larger than 1 and less than or equal to n, the ratio between width of the grooves of Step j-1 and that of Step j is 0.6-0.8. Additionally, the ratio between the width of sawteeth of Step j-1 and that of Step j is 0.5-0.7. When j equals to 2, the width of micro teeth of Step 1 is at least 0.5 mm, and the width of grooves is at least 0.3 mm; The sawteeth (1-1) and the grooves (1-2) are helically arranged around the drill axis, and the helix angle of sawteeth is acute angle larger than 60°. The bottom edges (1-3) of sawteeth perform as cutting edges, and the rake angle of the edges is ground into positive angle (1-γ0u). 